Microwave communications using digital modulation techniques require signal modulators which perform vector or quadrature modulation. In vector modulation two modulation input signals independently modulate the in-phase (I) and quadrature (Q) components of the carrier. For proper operation, the I and Q channels of the modulator must be calibrated to be equal in gain, i.e., balanced, and precisely 90.degree. apart, i.e., in quadrature.
One way to calibrate a vector modulator is with a network analyzer connected to the RF carrier input and to the RF modulated output. The network analyzer measures the amplitude and phase of the RF modulated output resulting from varying DC voltages applied to the I and Q modulation inputs. From these measurements, the gain and phase accuracy of the modulator can be determined, and calibrated. This is an expensive and complicated technique that is also limited in accuracy.
Structure and techniques for calibrating a vector modulator that does not require a network analyzer, are provided by the present invention and incorporated in a system comprising a standard vector modulator circuit including means for shifting phase to adjust the relative phases of the I and Q components of the modulated RF carrier. Means for variable attenuating are included in the I and Q modulation system to adjust the relative amplitude of the modulation signals.
The present invention provides a technique wherein the microwave transmitter's I and Q phase difference is measured by measuring the power levels of the I and the Q modulation signals. The power levels are compared and a correction value is stored in a read-only memory to compensate for the error.
More particularly, the error compensation is achieved by first determining amplitude error or unbalance by I and Q signal power measurement and an amplitude compensation signal is stored, then a power measurement is carried out for phase compensation and a phase compensation signal is stored. The amplitude and phase compensation signals are used to modify the I/Q digital input signals.
An object of the present invention is to provide an improved vector modulator having compensation of amplitude and phase errors by digital signal processing.
Another object of the present invention is to provide an improved vector modulator wherein the modulator is tuned by stored computed correction values.
A further object of the present invention is to provide a vector modulator wherein correction signals are generated by measuring the power of the modulator output signal and comparing them to stored correction values.